The separation of nucleated cells from complex biological fluid is a critical step in disease diagnostics, for profiling genetic and other biomolecule signatures of the patient and for generating a patient-specific treatment strategy. Recently, the field of inertial micro-fluidics has shown promise for in separating cells from a biofluid with high processing rates [1]. An inertial micro-fluidics cell sorting system is a micro-fluidic device which separates cells of a specific size by utilizing the inertial forces that particles experience in a micro-fluidic channel. Particles inside the micro-fluidic channel experience different magnitudes of forces depending on their sizes. There are two major forces acting on particles inside the curved microchannel, one being the net lift force and the other being the Dean drag force. The net lift force (FN) and the Dean drag force (FD) are proportional to the fourth power and the first power of the particle diameter (ap), respectively:FN=ρG2CLap4,FD=3πμUDeanap=5.4×10−4πμDe1.63ap,where ρ is the density of fluid medium, G=UMax/Dh, where UMax is the maximum fluid velocity and Dh is the microchannel hydraulic diameter, CL is the lift coefficient which is a function of the particle position across the channel cross-section and channel Reynolds number (Re), and De is Dean number which is given by
            De      =                                                  ρ              ⁢                                                          ⁢                              U                f                            ⁢                              D                h                                      μ                    ⁢                                                    D                h                                            2                ⁢                R                                                    =                  Re          ⁢                                                                      D                  h                                                  2                  ⁢                  R                                                      ⁡                          [              2              ]                                            )    .
The balance between these two forces in the curved microchannel geometry determines the position of a particle of a specific size in a flow with specific flow velocity. Since different-sized particles are transported to different positions of the channel, they can be separated out by appropriate bifurcation at the channel outlet. Unlike other micro-fluidic-based cell separation methods, such as acoustic, electroosmotic, electrophoretic and magnetic separation methods, inertial micro-fluidics have simple flow-pump setups and require no additional treatment and have no effect on the target cell-of-interest. In addition, the use of spiral micro-fluidics for biosample preparation prior to cell-based downstream assay, e.g. ELISA, PCR, [2-4] and bacterial detection, has been reported [5].
There remains a need for a technique for the efficient isolation and/or enrichment of particles and/or cells, such as rare cells.